Embodiments of the invention relate generally to switching devices for switching on/off a current in current paths, and more particularly to micro-electromechanical system based switching devices.
To switch on/off current in electrical systems, a set of contacts may be used. The contacts may be positioned as open to stop current, and closed to promote current flow. Generally, the set of contacts may be used in contactors, circuit-breakers, current interrupters, motor starters, or similar devices. However, the principles of switching current on/off may be understood through explanation of a contactor.
A contactor is an electrical device designed to switch an electrical load ON and OFF on command. Traditionally, electromechanical contactors are employed in control gear, where the electromechanical contactors are capable of handling switching currents up to their interrupting capacity. Electromechanical contactors may also find application in power systems for switching currents. However, fault currents in power systems are typically greater than the interrupting capacity of the electromechanical contactors. Accordingly, to employ electromechanical contactors in power system applications, it may be desirable to protect the contactor from damage by backing it up with a series device that is sufficiently fast acting to interrupt fault currents prior to the contactor opening at all values of current above the interrupting capacity of the contactor.
Previously conceived solutions to facilitate use of contactors in power systems include vacuum contactors, vacuum interrupters and air break contactors, for example. Unfortunately, contactors such as vacuum contactors do not lend themselves to easy visual inspection as the contactor tips are encapsulated in a sealed, evacuated enclosure. Further, while the vacuum contactors are well stated for handling the switching of large motors, transformers, and capacitors, they are known to cause undesirable transient overvoltages, particularly as the load is switched off.
Furthermore, the electromechanical contactors generally use mechanical switches. However, as these mechanical switches tend to switch at a relatively slow speed, predictive techniques are employed in order to estimate occurrence of a zero crossing, often tens of milliseconds before the switching event is to occur, in order to facilitate opening/closing near the zero crossing for reduced arcing. Such zero crossing prediction is prone to error as many transients may occur in this prediction time interval.
As an alternative to slow mechanical and electromechanical switches, fast solid-state switches have been employed in high speed switching applications. As will be appreciated, these solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, because solid-state switches do not create a physical gap between contacts as they are switched into a non-conducing state, they experience leakage current. Furthermore, due to internal resistances, if solid-state switches operate in a conducting state, they experience a voltage drop. Both the voltage drop and leakage current contribute to the generation of excess heat under normal operating circumstances, which may affect switch performance and life. Moreover, due at least in part to the inherent leakage current associated with solid-state switches, their use in circuit breaker applications is not practical.
Furthermore, switching currents on or off during current flow may produce arcs, or flashes of electricity, which are generally undesirable. As described above, contactors may switch alternating current (AC) near or at a zero-crossing point where current flow is reduced compared to other points on an alternating current sinusoid. In contrast, direct current (DC) typically does not have a zero-crossing point. As such, arcs may occur at any instance of interruption.
Therefore, direct current interruption imposes different switching requirements compared to alternating current interruption. For example, if there is a significant amount of current or voltage, an alternating current interrupter may wait for an AC sinusoidal load or fault current to reach a naturally occurring zero before interruption. In contrast, DC interrupters do not experience a naturally occurring zero, and therefore must force a lower current or voltage in order to reduce arcing. Electronic devices such as transistors or field-effect transistors may force DC current to lower levels, but have the drawback of having high conducting voltage drop and power losses.
Accordingly, there exists a need in the art for a direct current control device and/or interrupter arrangement to overcome these drawbacks.